Small Form Factor Pluggable Double Density (SFP-DD) and Quad Small Form Factor Pluggable Double Density (QSFP-DD) are compact, pluggable electro-optical transceiver systems specified by the SFP-DD and QSFP-DD Multi-Source Agreement (MSA) groups. SFP-DD transceivers support two differential signal lanes, whereas QSFP-DD transceivers support eight differential signal lanes. SFP-DD is backward compatible to SFP, and QSFP-DD is backward compatible to QSFP.
SFP-DD is specified, for example, in “SFP-DD MSA SFP-DD Hardware Specification for SFP Double Density 2× Pluggable Transceiver,” Rev 1.1, Jan. 7, 2018, which is incorporated herein by reference. QSFP-DD is specified, for example, in “QSFP-DD MSA QSFP-DD Hardware Specification for QSFP Density 8× Pluggable Transceiver,” Rev 3.0, Sep. 19, 2017, which is incorporated herein by reference.